It is well-known that the state or degree of residual stresses present in a machine part subject to bending or contact loading can have a major influence on its service life. Much effort has been devoted toward developing compressive surface residual stresses by shot peening, surface rolling and by heat treatments such as carburizing, carbonitriding and nitriding. Descriptions of many of these methods are included in the following publications which are incorporated herein by reference:
(1) J. O. Alman and P. H. Black, "Residual Stress and Fatigue in Metals", McGraw-Hill Book Co., New York, 1963, Chapters 5 and 14.
(2) G. M. Rassweiler and W. L. Grube (Editors), "Internal Stresses and Fatigue in Metals", Elsevier Publishing Co., New York, 1959, pp. 110-119.
(3) Metals Handbook, Vol. 11, 8th Edition, American Society for Metals, Metals Park, Ohio, 1964, "Case Hardening of Steel".
(4) Carburizing and Carbonitriding", American Society for Metals, Metals Park, Ohio, 1977, pp. 86-92.
Each of the above known methods of developing compressive surface residual stresses when applied to high carbon steels have their attendant disadvantages. For example, shot peening and surface rolling are disadvantageous because of limitations as to (a) material hardness, (b) size and shape of part, and (c) resulting surface finish that cannot meet all requirements. Nitriding at temperatures of 1100.degree. F. and below is usually economical only as a shallow surface treatment and therefore disadvantageous. Carbonitriding or nitriding, while steel is in an austenitic condition, requires simultaneous control of both carbon and nitrogen potentials in the gas phase; it is difficult to accurately control the potentials and therefore it is frequently overdone, producing high levels of retained austenite along the part surface which is disadvantageous. As to carburizing, it is generally assumed that it is not possible, by diffusing more carbon into the surface of a high carbon alloyed steel, to produce compressive residual surface stresses (see 14th International Colloqium on Heat Treating, 1972, p. 11).
Carburizing techniques are nearly always applied to low carbon, low alloy steels, such as AISI 8620, 4118 and 4620, which contain 0.1-0.3 wt. pct. carbon. For hypereutectoid steels, austenitized at temperatures too low to dissolve all carbides, an effective equilibrium is established between undissolved carbide and the austenite, which is then saturated in carbon. It is also generally accepted that this saturation prevents such steel from accepting additional dissolved carbon, and thereby prevents an increase in the amount of carbon dissolved in the austenite near the surface. Therefore, an appreciation of carburization with respect to hypereutectoid alloy steels, has remained an unexplored area until this invention.
This is not to say that the prior art has not employed heat treatment methods to produce compressive residual surface stresses in hypereutectoid alloy steels, but they have been carried out by methods which have required the addition of ammonia to the austenitizing furnace atmosphere, which in turn causes nitrogen to be dissolved in the surface layers of the steel. The goal of using such atmosphere is to increase the nitrogen content of the austenite surface, but at the same time avoiding the formations of nitrides of iron or other alloying elements. Ammonia atmospheres present special equipment requirements which it is desirable to avoid and present control problems as to nitride avoidance.